Concentrated fluoropolymer dispersions stabilized with anionic polyelectrolyte dispersing agents

ABSTRACT

An aqueous fluoropolymer dispersion comprising fluoropolymer particles having an average particle size of about 10 to about 400 nm. The dispersion has a solids content of at least about 35 to about 70 wt % and comprises about 0.03 wt % to about 10 wt % anionic polyelectrolyte dispersing agent based on the weight of fluoropolymer solids. The dispersion has a Gel Time of at least about 100 seconds.

FIELD OF INVENTION

The present invention relates to concentrated fluoropolymer dispersionsand more particularly to concentrated stabilized dispersions withreduced fluorosurfactant content.

BACKGROUND OF THE INVENTION

Fluoropolymers are applied to a wide number of substrates in order toconfer release, chemical and heat resistance, corrosion protection,cleanability, low flammability, and weatherability. Coatings ofpolytetrafluoroethylene (PTFE) homopolymers and modified PTFE providethe highest heat stability among the fluoropolymers, but unliketetrafluoroethylene (TFE) copolymers, cannot be melt processed to formfilms and coatings. Therefore, other processes have been developed forapplying coatings of PTFE homopolymers and modified PTFE. One suchprocess is dispersion coating which applies the fluoropolymer indispersion form. Dispersions used in coating processes are usually in aconcentrated form and contain a significant quantity of nonionicsurfactant, e.g. 6-8 weight percent, as taught in Marks et al., U.S.Pat. No. 3,037,953. Similar dispersions and coating processes are alsoused for making coatings of melt-processible fluoropolymers.

For some specialized fluoropolymer coating dispersions, common nonionicsurfactants such as alkyl phenol ethoxylates or aliphatic alcoholsethoxylates for stabilization are unsuitable. One such dispersion is aformulation in which chromic acid is used as a component of a primercoating to increase adherence to metal substrates. Chromic acid has beenshown to attack nonionic surfactants such as TRITON® X-100, an alkylphenol ethoxylate. Historically, an anionic surfactant such as sodiumlauryl sulfate has been used in this primer formulation because it isstable in the presence of chromic acid.

Fluorosurfactants are typically used as an ingredient in the dispersionpolymerization of fluoropolymers, the fluorosurfactants functioning as anon-telogenic dispersing agent. For example, an early description ofthis use of fluorosurfactants is found in U.S. Pat. No. 2,559,752 toBerry. However because of environmental concerns and becausefluorosurfactants are expensive, processes have been developed forreducing and recovering fluorosurfactant from aqueous fluoropolymerdispersions.

One common method is to remove fluorosurfactant by adsorption onto anion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) andU.S. Pat. No. 4,282,162 (Kuhls) and U.S. Pat. No. 6,833,403 (Bladel etal.) For effective removal, such dispersions are stabilized with anonionic surfactant, such as alkyl phenol ethoxylates or aliphaticalcohol ethoxylates as disclosed in U.S. Pat. No. 3,037,953 to Marks etal.; U.S. Pat. No. 6,153,688 to Miura et al.; and U.S. Pat. No.6,956,078 to Cavanaugh et al. Dispersions stabilized with nonionicsurfactant are used since removal of the fluorosurfactant withoutnonionic surfactant being present generally results in coagulation ofthe dispersion.

If it is attempted to use an anion exchange process for thefluoropolymer dispersions discussed above that are stabilized with ananionic hydrocarbon surfactant such as sodium lauryl sulfate instead ofnonionic surfactant, the anionic hydrocarbon surfactant will be removedfrom the dispersion together with the fluorosurfactant causingcoagulation of the dispersion.

Improved stabilized concentrated fluoropolymer dispersions with reducedfluorosurfactant content are desired.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an aqueous fluoropolymer dispersioncomprising fluoropolymer particles having an average particle size ofabout 10 to about 400 nm. The dispersion has a solids content of atleast about 35 to about 70 wt % and comprises about 0.03 wt % to about10 wt % anionic polyelectrolyte dispersing agent based on the weight offluoropolymer solids. The dispersion has a Gel Time of at least about100 seconds.

In a preferred embodiment of the dispersion of the invention, theanionic polyelectrolyte dispersing agent has an equivalent weightgreater than about 150. Preferably, the equivalent weight of the anionicpolyelectrolyte dispersing agent is less than about 50,000. The anionicpolyelectrolyte dispersing agent preferably has a number averagemolecular weight of at least about 500. One preferred embodiment employsan acrylic copolymer dispersing agent having a molecular weight of about2000 to about 100,000. Preferably, the dispersion comprises less thanabout 300 ppm fluorosurfactant based on the weight of the dispersion.

The invention further provides a process for reducing thefluorosurfactant content of and concentrating an aqueousfluorosurfactant-containing fluoropolymer dispersion comprisingstabilizing fluoropolymer dispersion comprising fluoropolymer particleshaving an average size of about 10 to about 400 nm and a solids contentof about 15 to about 55 wt % with anionic polyelectrolyte dispersingagent, contacting the stabilized fluorosurfactant-containing aqueousfluoropolymer dispersion with an anion exchange resin to reducefluorosurfactant content to a predetermined level, separating the anionexchange resin from the dispersion after the fluorosurfactant contenthas been reduced to produce a reduced-fluorosurfactant dispersion, andconcentrating the dispersion to at least about 35% by weight using amethod which produces dispersion substantially free of nonionicsurfactants.

The invention further relates to a process for applying fluoropolymer toa substrate comprising providing a concentrated aqueous fluoropolymerdispersion comprising fluoropolymer particles and having a solidscontent of about 35 to about 70 wt %, the dispersion comprising about0.03 wt % to about 10 wt % anionic polyelectrolyte dispersing agentbased on the weight of fluoropolymer solids and containing less thanabout 300 ppm fluorosurfactant based on the weight of the dispersion,the dispersion being substantially free of nonionic dispersion; applyingthe dispersion to the substrate; and heating the substrate coated withdispersion to coalesce the fluoropolymer particles on the substrate.

DETAILED DESCRIPTION OF THE INVENTION Fluoropolymers

The aqueous fluoropolymer dispersion in accordance with the presentinvention is made by dispersion polymerization (also known as emulsionpolymerization). Fluoropolymer dispersions are comprised of particles ofpolymers made from monomers wherein at least one of the monomerscontains fluorine. The fluoropolymer of the particles of the aqueousdispersions of this invention is independently selected from the groupof polymers and copolymers of trifluoroethylene, hexafluoropropylene,monochlorotrifluoroethylene, dichlorodifluoroethylene,tetrafluoroethylene, perfluoroalkyl ethylene monomers, perfluoro(alkylvinyl ether) monomers, vinylidene fluoride, and vinyl fluoride.

Preferred fluoropolymer particles used in the dispersion employed inthis invention are non-melt-processible particles ofpolytetrafluoroethylene (PTFE) including modified PTFE which is notmelt-processible. Polytetrafluoroethylene (PTFE) refers to thepolymerized tetrafluoroethylene by itself without any significantcomonomer present. Modified PTFE refers to copolymers of TFE with suchsmall concentrations of comonomer that the melting point of theresultant polymer is not substantially reduced below that of PTFE. Theconcentration of such comonomer is preferably less than 1 wt %, morepreferably less than 0.5 wt %. A minimum amount of at least about 0.05wt % is preferably used to have significant effect. The modified PTFEpreferably contains a comonomer modifier which improves film formingcapability during baking (fusing), such as perfluoroolefin, notablyhexafluoropropylene (HFP) or perfluoro(alkyl vinyl)ether (PAVE), wherethe alkyl group contains 1 to 5 carbon atoms, with perfluoro(ethylvinyl)ether (PEVE) and perfluoro(propyl vinyl)ether (PPVE) beingpreferred. Chlorotrifluoroethylene (CTFE), perfluorobutyl ethylene(PFBE), or other monomer that introduces bulky side groups into themolecule are also included. In this preferred form of the invention, thePTFE typically has a melt creep viscosity of at least 1×10⁹ Pa·s. Theresins in the dispersion used in this form of the invention whenisolated and dried are thus non-melt-processible.

By non-melt-processible, it is meant that no melt flow is detected whentested by the standard melt viscosity determining procedure formelt-processible polymers. This test is according to ASTM D-1238-00modified as follows: The cylinder, orifice and piston tip are made ofcorrosion resistant alloy, Haynes Stellite 19, made by Haynes StelliteCo. The 5.0 g sample is charged to the 9.53 mm (0.375 inch) insidediameter cylinder which is maintained at 372° C. Five minutes after thesample is charged to the cylinder, it is extruded through a 2.10 mm(0.0825 inch diameter), 8.00 mm (0.315 inch) long square-edge orificeunder a load (piston plus weight) of 5000 grams. This corresponds to ashear stress of 44.8 KPa (6.5 pounds per square inch). No melt extrudateis observed.

In one preferred embodiment, the fluoropolymer particles in thedispersion used in this invention comprise a core of high molecularweight polytetrafluoroethylene (PTFE) and a shell of lower molecularweight polytetrafluoroethylene or modified polytetrafluoroethylene.

The preferred non-melt-processible PTFE or modified PTFE have a standardspecific gravity (SSG) of about 2.13 to about 2.50. Preferably, the SSGis less than about 2.40, more preferably less than about 2.30, and mostpreferably less than about 2.25. The SSG is generally inverselyproportional to the molecular weight of PTFE or modified PTFE.

The fluoropolymer particles in the dispersion used in this inventionhave a number average particle size of about 10 nm to about 400 nm,preferably, about 100 nm to about 350 nm.

A typical process for the aqueous dispersion polymerization of preferredPTFE polymer is a process wherein TFE vapor is fed to a heated reactorcontaining fluorosurfactants, paraffin wax and deionized water. A chaintransfer agent may also be added if it is desired to reduce themolecular weight of the PTFE. A free-radical initiator solution is addedand, as the polymerization proceeds, additional TFE is added to maintainthe pressure. The exothermic heat of reaction is removed by circulatingcooling water through the reactor jacket. After several hours, the feedsare stopped, the reactor is vented and purged with nitrogen, and the rawdispersion in the vessel is transferred to a cooling vessel. Paraffinwax is removed and the dispersion is isolated and stabilized withdispersing agent.

The fluorosurfactant used in the manufacture of the dispersion is anon-telogenic, anionic dispersing agent, soluble in water and comprisingan anionic hydrophilic group and a hydrophobic portion. Preferably, thehydrophobic portion is an aliphatic fluoroalkyl group containing atleast four carbon atoms and bearing fluorine atoms and having no morethan two carbon atoms not bearing fluorine atoms adjacent to thehydrophilic group. These fluorosurfactants are used as a polymerizationaid for dispersing and, because they do not chain transfer, they do notcause formation of polymer with undesirable short chain length. Anextensive list of suitable fluorosurfactants is disclosed in U.S. Pat.No. 2,559,752 to Berry. Preferably, the fluorosurfactant is aperfluorinated carboxylic or sulfonic acid having 6-10 carbon atoms andis typically used in salt form. Suitable fluorosurfactants are ammoniumperfluorocarboxylates, e.g., ammonium perfluorocaprylate or ammoniumperfluorooctanoate. The fluorosurfactants are usually present in theamount of 0.02 to 1 wt % with respect to the amount of polymer formed.The fluorinated surfactant is used to aid the polymerization process butthe amount remaining in the dispersion is significantly reduced as willbe explained below.

The initiators, preferably used to make dispersion of this invention arefree radical initiators. They may be those having a relatively longhalf-life, preferably persulfates, e.g., ammonium persulfate orpotassium persulfate. To shorten the half-life of persulfate initiators,reducing agents such as ammonium bisulfite or sodium metabisulfite, withor without metal catalysis salts such as Fe (III), can be used.Alternatively, short half-life initiators such as potassiumpermanganate/oxalic acid can be used.

In addition to the long half-life persulfate initiators, small amountsof short chain dicarboxylic acids such as succinic acid or initiatorsthat produce succinic acid such as disuccinic acid peroxide (DSP) may bealso be added in order to reduce coagulum

To produce dispersion with low fluorosurfactant content as describedbelow, sufficient anionic polyelectrolyte dispersing agent as isdescribed in more detail hereinafter is added to prevent coagulation ofthe dispersion when the fluorosurfactant content is reduced. Typically,dispersing agent is added for stabilization prior to fluorosurfactantreduction

The dispersion is concentrated as will be described below. Theconcentrated aqueous dispersion has a fluoropolymer solids content of atleast about 35 wt %, preferably at least about 40 wt %, and morepreferably at least about 45 wt %. The concentrated aqueous dispersioncan range in fluoropolymer solids content from at least about 35 wt % toabout 70 wt %, preferably at least about 40 wt % to about 70 wt %, morepreferably at least about 45 wt % to about 70 wt % and especially atleast about 50 wt % to about 70 wt %.

The aqueous dispersion in accordance with the invention is substantiallyfree of nonionic surfactants. “Substantially free” means that thedispersion contains either no nonionic surfactant or so little nonionicsurfactant that such nonionic surfactant will not interfere with end-useapplications. Preferably, the dispersion contains less than about 2 wt %of nonionic surfactant based on dispersion weight, preferably less thanabout 1 wt %, more preferably less than about 0.5 wt % and especiallyless than 0.1 wt %.

Dispersing Agents

Fluoropolymer dispersions are typically stabilized with surfactants. Asurfactant has a hydrophilic portion and a hydrophobic portion on thesame molecule. These can be either cationic, nonionic or anionic. Atypical cationic surfactant has a positively charged hydrophilic portionsuch as an alkylated ammonium halide and a hydrophobic portion such as along chain fatty acid. An anionic surfactant has a negatively chargedhydrophilic portion such as a carboxylate or sulfate salt and a longchain hydrocarbon portion as the hydrophobic portion. A nonionicsurfactant does not contain a charged group but has a hydrophobicportion that is typically a long chain hydrocarbon similar to the othertwo types of surfactants. The hydrophilic portion of the nonionicsurfactant typically contains water soluble functionality such as achain of ethylene ether derived from polymerization with ethylene oxide.Water solubility is due to hydrogen bonding of the ether oxygen atomswith protons from the water. Surfactants stabilize polymer particles bycoating the particles with the hydrophobic portion of the surfactantoriented towards the particle and the hydrophilic portion of thesurfactant in the water phase. In the case of charged surfactants, somestability is also due to repulsion of the electrical charges betweenparticles. Surfactants typically reduce surface tension significantlyand allow better wetting of surfaces with the dispersion. In contrast,anionic polyelectrolyte dispersing agents are employed in dispersions inaccordance with the invention for stabilization. These dispersing agentsare different from surfactants in that they do not contain distincthydrophilic and hydrophobic portions. It is believed that stabilizationof fluoropolymer dispersions occurs when the anionic polyelectrolytedispersing agents coat the fluoropolymer particles. The anionic groupscarried on the dispersing agents increase the surface charge on theparticle and confer stability by repulsion of the electrical chargesbetween particles. Unlike surfactants, these dispersing agents typicallyhave little if any significant effect on surface tension of thedispersion. The surface tension of the dispersion containing anionicpolyelectrolyte dispersing agents remains high unless surfactants areadded to alter wetting properties, for viscosity control, improvestability, etc. Nonionic surfactants, substantially reduce the surfacetension of the aqueous solutions. In contrast the polyelectrolytes areoften used as flocculating agents of colloidal dispersions whereassurfactants are normally used to stabilize colloidal dispersions.Surface tension of a preferred dispersion in accordance with the presentinvention is greater than about 35 dyne/cm at 25° C., preferably greaterthan about 40 dyne/cm, still more preferably greater than about 45dyne/cm.

The anionic polyelectrolyte dispersing agents employed in accordancewith dispersions and processes of the present invention are preferablyanionic polymers, having a linear or branched structure, with anionicgroups distributed along the polymeric chain, optionally present also inthe chain end groups. The polyelectrolytes preferably have an equivalentweight, defined as molecular weight/number of anionic groups present inthe polyelectrolyte, greater than about 150, preferably greater thanabout 200, still more preferably greater than about 250. Generally theequivalent weight of the anionic polyelectrolyte dispersing agentsusable in the process of the present invention is less than about50,000, preferably less than about 10,000, more preferably less thanabout 3,000, still more preferably less than about 1,500.

The number average molecular weight of the anionic polyelectrolytedispersing agent is preferably at least about 500, more preferably inthe range of about 500 to about 100,000. More preferably, the molecularweight is at least about 1,000. Especially preferred embodiments have amolecular weight of about 2,000 to about 100,000 and preferably 5,000 toabout 20,000.

The anionic polyelectrolyte dispersing agents usable in the processaccording to the present invention preferably contain in the molecule anumber of anionic functional groups higher than or equal to 2, morepreferably greater than or equal to 5. The anionic groups present in themolecule of the anionic polyelectrolyte agents are preferably selectedfrom carboxylates, sulphates, sulphonates, phosphates, phosphonates, aremore preferably carboxylates, sulphates, sulphonates, and still morepreferably are carboxylates. Generally, the anionic polyelectrolytedispersing agents do not contain fluorine atoms.

Preferably, the anionic polyelectrolyte dispersing agents used accordingto the present invention are selected from anionic homopolymers orcopolymers of monomers selected from acrylic or vinyl monomers whichpreferably provide a number of anionic groups as mentioned above givethe equivalent weight as defined above. Preferably, acrylic monomers areselected from (meth)acrylamide, (meth)acrylic acid in the form of thecorresponding salts, (meth)acrylonitrile, linear or branched C₁-C₄hydroxyesters of the (meth)acrylic acid, C₁-C₁₂ alkyl(meth)acrylates,wherein the alkyl can be linear or branched, compounds of the followinggeneral formula:

wherein: R₁ is H or CH₃; R₂ and R₃, equal or different, are H or C₁-C₈alkyl, optionally branched; M is an alkaline or alkaline-earth metal orammonium and A is NH, O or NCH₃.

Among the vinyl monomers which can provide anionic polyelectrolytes foruse in accordance with the present invention, vinylaromatic monomers canbe used, preferably styrene and its derivatives obtained by substitutingone or more hydrogen atoms of the aromatic ring with a hydroxyl or amethyl and/or of vinyl with a methyl, for example, alpha-methylstyrene;C₁-C₁₂ alkyl vinylethers, such as methyl-, ethyl-, n-propyl-,isopropyl-, n-butyl-, isobutyl- and 2-ethylhexyl-vinyl ether; and vinylesters of C₁-C₁₈ aliphatic monocarboxylic acids, such as vinyl acetate,vinyl propionate, vinyl butyrate, vinyl-2-ethyl-hexanoate, and vinylstearate.

Homopolymers or copolymers of one or more monomers selected from acrylicor vinyl monomers are obtainable by aqueous suspension polymerization byradical or ionic addition, according to well known methods of the priorart. See for example Kirk Othmer “Encyclopedia of Chemical Technology”,III Ed., vol. 18, pages 720-744. In case of radical polymerization inaqueous suspension, as radical initiators, those soluble in monomers arepreferably used and furthermore suspending agents, surfactants are used.

As radical initiators, aliphatic and aromatic peroxides are for exampleused, as for example t-butylperoxy-2-ethylhexanoate, dibenzoylperoxide,benzoylperoxide, laurylperoxide, t-butylperoxydiethylacetate or unstableazocompounds as for example azodiisobutyronitrile. In the monomericmixture also a chain transfer agent can optionally be used. Mercaptancompounds, as mercaptoethanol, mercaptopropanol, mercaptobutanol,mercaptoacetic acid, mercaptopropionic acid, butylmercaptan,n-dodecylmercaptan, can for example be mentioned. The polymerizationtemperatures are those at which there is the initiator decomposition,and are generally from about 50° C. to about 120° C. For the suspendingagents see for example EP 457,356.

Other usable anionic polyelectrolyte dispersing agents are polyamicacids, preferably aromatic polyamic acids or polyamidoamic acids.Examples of repeating units of these polymers are:

amido-amic acid:

amidoimidic unit:

wherein R^(II) is a divalent arylene radical. See, for example, U.S.Pat. No. 6,479,581 which describes preparation of such polymers.

Other usable anionic polyelectrolyte dispersing agents are carboxyalkylcelluloses, wherein the alkyl comprises from 1 to 5 carbon atoms,preferably from 1 to 3, for example carboxymethylcellulose.

The polyelectrolyte dispersing agents usable in accordance with theinvention are, for example, those sold under the trademarks Craymul®8212 (Cray Valley), Torlon® AI30 (Solvay Advanced Polymers), Torlon®AI50 (Solvay Advanced Polymers), Elvacite® 2669 and Elvacite® 2776(Lucite International), and Joncryl® DFC 3025 (Johnson Polymer).

Anionic polyelectrolytes for used in accordance with the presentinvention are generally soluble in water. Co-solvents miscible withwater such as alcohols, e.g., isopropyl alcohol, ketones, e.g.,N-methylpyrrolidone, can optionally be added.

The anionic polyelectrolyte dispersing agent is added in an amount ofabout 0.03 to about 10 wt %, preferably about 0.1 wt % to about 10 wt %,more preferably from about 0.2 wt % to about 5 wt %, still morepreferably from about 0.5 wt % to about 3 wt % in percent by weightbased on the weight of the fluoropolymer solids. The polyelectrolyteamount generally depends on the type of polyelectrolyte used. Oneskilled in the art is easily able determine an appropriate amountsufficient to confer the stability desired.

One class of preferred anionic dispersing agents are acrylic copolymers,more preferably acrylic copolymer dispersing agents described as beinghydrophobic acrylic copolymers. Examples of polymer of this type aresold under the trademarks TAMOL® 681, TAMOL® 2001, TAMOL® 165A andTAMOL® 731A sold by Rohm and Haas. Such dispersing agents are known foruse in acrylic-based paints to prevent agglomeration of pigments butthey are not known for use in fluoropolymer dispersions. A preferredacrylic copolymer dispersing agent for use in this invention comprisesmethacrylic acid/butyl methacrylate copolymer. More preferably themethacrylic acid/butyl methacrylate copolymer comprises about 30 toabout 50 mol % methacrylic acid units and about 50 to about 70 mol %butyl methacrylate units. In embodiments of the invention employing thistype of acrylic copolymer anionic polyelectrolyte, amounts acryliccopolymer of about 0.5 wt % to about 5.5 wt % based on the weight offluoropolymer solids have been found to be especially useful.Percentages of the acrylic copolymer dispersing agent are based onactive ingredients.

A preferred acrylic copolymer dispersing agent for this invention has amolecular weight of about 2,000 to about 100,000 and more preferably hasa molecular weight of about 5,000 to about 20,000.

Although the acrylic copolymer dispersing agent may be supplied in acidform, it is employed in salt form in the fluoropolymer dispersions ofthe invention for effective stabilization. Although various salt formscan be used, a preferred form of the acrylic copolymer dispersing agentis in the form of an ammonium salt so that it does not introduceextraneous cations into the dispersion. For the acrylic copolymerdispersing agent to be predominantly in salt form and soluble in water,the pH of the fluoropolymer dispersion is preferably at least about 9,more preferably at least about 9.5.

Dispersion Shear Stability—Gel Time

In accordance with the invention, the dispersion has a Gel Time of atleast about 100 seconds as determined by the Gel Time test described inthe Test Methods of this application. Gel Time is a measurement ofresistance of the dispersion to coagulation under high shear conditionsand thus is an indicator of the stability of the dispersion duringprocessing which subjects the dispersion to shear. Although affected bya variety of factors including solids content, pH, molecular weight ofthe polymer, polymer particle morphology, other materials in thedispersion, etc., a Gel Time of at least 100 indicates that the anionicpolyelectrolyte dispersing agent is functioning to stabilize the polymersufficiently for normal handling and processing, e.g., is sufficientlystabilized for fluorosurfactant removal in an anion exchange column.More preferably, the Gel Time is at least about 300 seconds, even morepreferably at least about 500 seconds, even more preferably at leastabout 1000 seconds, and most preferably at least about 1500 seconds. Apreferred range of Gel Time provided by the present invention is about100 seconds to about 2000 seconds. In accordance with a preferred formof the invention, the dispersion contains less than about 300 ppmfluorosurfactant based on the weight of the dispersion and has the GelTimes as indicated above. Preferably, the Gel Times described above areobserved when the fluorosurfactant content is less than about 100 ppm,most preferably less than about 50 ppm.

Hydrophobic acrylic copolymer anionic polyelectrolytes described aboveare especially useful in accordance with the invention for providingdesirable gel times.

Fluorosurfactant Reduction

In accordance with the process of the invention for reducing thefluorosurfactant content of and concentrating afluorosurfactant-containing aqueous fluoropolymer dispersion, thefluorosurfactant content of the dispersion stabilized with anionicpolyelectrolyte dispersing agent is reduced to a predetermined level.The resulting dispersion has reduced fluorosurfactant content than aspolymerized dispersion and preferably has a fluorosurfactant content ofless than about 300 ppm based on the total dispersion weight.Preferably, the fluorosurfactant content is less than about 100 ppm,more preferably less than about 50 ppm.

The fluorosurfactant content can be reduced by any of a variety ofprocedures as known in the art. In the preferred embodiment of thepresent invention, the fluorosurfactant is removed by adsorption onto ananion exchange resin. Contacting of the dispersion with anion exchangeresin is preferably performed before concentration especially when apreferred concentration method is employed using the addition of anacrylic polymer with high acid content described in U.S. Pat. No.5,272,186 to Jones because the acrylic polymer with high acid contentmay be adsorbed onto the anion exchange resin.

Any of a variety of techniques which bring the dispersion in contactwith the anion exchange resin can be used to carry out ion exchange ofthe process. For example, the process can be carried out by addition ofion exchange resin bead to the dispersion in a stirred tank, in which aslurry of the dispersion and resin is formed, followed by separation ofdispersion from the anion exchange resin beads by filtration. Anothersuitable method is to pass the dispersion through a fixed bed of anionexchange resin instead of using a stirred tank. Flow can be upward ordownward through the bed and no separate separation step is needed sincethe resin remains in the fixed bed.

The contacting of the dispersion is performed at a temperature which issufficiently high to facilitate the rate of ion exchange and to reducethe viscosity of the dispersion but being below a temperature at whichthe resin degrades at a detrimentally high rate or a viscosity increasein observed. Upper treatment temperature will vary with the type ofpolymer and nonionic surfactant employed. Typically, temperatures willbe between 20° C. and 80° C.

The fluorosurfactant can be recovered from the anion exchange resin ifdesired or the resin with the fluorosurfactant can be disposed of in anenvironmentally acceptable method, e.g., by incineration. If it isdesired to recover the fluorosurfactant, the fluorosurfactant may beremoved from resin by elution. Elution of fluorosurfactant adsorbed onthe anion exchange resin is readily achieved by use of ammonia solutionas demonstrated by Seki in U.S. Pat. No. 3,882,153, by a mixture ofdilute mineral acid with organic solvent (e.g., HCl/ethanol) asdemonstrated by Kuhls in U.S. Pat. No. 4,282,162, or by strong mineralacids such as sulfuric acid and nitric, transferring the adsorbedfluorinated carboxylic acid to the eluent. The fluorosurfactant in theeluent in high concentration can easily be recovered in the form of apure acid or in the form of salts by common methods such asacid-deposition, salting out, and other methods of concentration, etc.

Ion Exchange Resins

The ion exchange resins for use in reducing the fluorosurfactant contentof the aqueous dispersion include anionic resins but can also includeother resin types such as cationic resins, e.g., in a mixed bed. Theanionic resins employed can be either strongly basic or weakly basic.Suitable weakly basic anion exchange resins contain primary, secondaryamine, or tertiary amine groups. Suitable strongly basic anion exchangeresin contain quaternary ammonium groups. Although weakly basic resinsare useful because they can be regenerated more easily, strongly basisresins are preferred when it is desired to reduce fluorosurfactant tovery low levels and for high utilization of the resin. Strongly basicion exchange resins also have the advantage of less sensitivity to thepH of the media. Strong base anion exchange resins have an associatedcounter ion and are typically available in, chloride or hydroxide formbut are readily converted to other forms if desired. Anion exchangeresins with hydroxide, chloride, sulfate, and nitrate can be used forthe removal of the fluorosurfactant but anion exchange resins in theform of hydroxide are preferred to prevent the introduction ofadditional anions and to increase pH during anion exchange because ahigh pH, i.e., greater than 9, is desirable in the product prior toshipping to inhibit bacterial growth. Examples of suitablecommercially-available strong base anion exchange resins with quaternaryammonium groups with a trimethylamine moiety include DOWEX® 550A, USFilter A464-OH, SYBRON M-500-OH, SYBRON ASB1-OH, PUROLITE A-500-OH,Itochu TSA 1200, AMBERLITE® IR 402. Examples of suitablecommercially-available strong base anion exchange resins with quaternaryammonium groups with a dimethyl ethanol amine moiety include US FilterA244-OH, AMBERLITE® 410, DOWEX® MARATHON A2, and DOWEX® UPCORE Mono A2.

Ion exchange resin used to reduce fluorosurfactant for use in theprocess is preferably monodisperse. Preferably, the ion exchange resinbeads have a number average size distribution in which 95% of the beadshave a diameter within plus or minus 100 μm of the number average beaddiameter.

Concentration

Concentration is carried out in accordance with the invention using amethod which provides a concentrated dispersion substantially free ofnonionic surfactants. Nonionic surfactants, such as alkyl phenolethoxylates or aliphatic alcohols ethoxylates, typically used forstabilization of fluoropolymer dispersions have a cloud point betweenabout 30° C. and about 90° C. which enable concentration by a phaseseparation process such as that disclosed in U.S. Pat. No. 3,037,953 toMarks et al. Anionic polyelectrolyte dispersing agents, on the otherhand, typically have no significant cloud point between about 30° C. andabout 90° C. Therefore, other methods are used in accordance with theinvention to provide the desired concentration which leaves theconcentrated dispersion free of nonionic surfactants.

One preferred method for concentration employs acrylic polymers of highacid content as described in U.S. Pat. No. 5,272,186 to Jones. Theconcentrating is performed by adding acrylic polymer of acid content of20% or more by weight to the reduced-fluorosurfactant dispersion in anamount of about 0.01 to about 1 wt % based on aqueous content of thedispersion and subjecting the dispersion to conditions causing thedispersion to separate into a lower phase high in fluoropolymer solidsand an upper phase low in fluoropolymer solids and recovering the lowerphase as concentrated reduced fluorosurfactant dispersion.

To carry out the preferred concentration process, U.S. Pat. No.5,272,186 states that the pH of the dispersion should be adjusted to atleast about 6. As discussed above, it is preferable for the pH of thedispersion to be at least about 9, more preferably at least 9.5 foreffective stabilization by the preferred anionic polyelectrolytedispersing agent so that adjustment usually will not be necessary. If pHadjustment is necessary, typically a base such as ammonium hydroxide isused. The acrylic polymer concentrating agent preferably used inconcentration has an acid content of 20% or more by weight. Preferably,the acrylic polymer of high acid content has a weight-average molecularweight of about 50,000 to about 1,000,000, and is employed in an amountof about 0.01 wt % to about 0.5 wt % based on the weight of the aqueousportion of the dispersion, more preferably about 0.02 to about 0.4 wt %.In a preferred embodiment, the acrylic polymers have a molecular weightof about 200,000 to 1,000,000. In another preferred embodiment theacrylic polymer is added in amount of about 0.03 to about 0.2 wt % basedon the weight of the aqueous portion of the dispersion. An especiallypreferred acrylic polymer of high acid content is polyacrylic acid.

After addition of the acrylic polymer having an acid content of 20% ormore by weight, the dispersion is subjected to conditions which cause aphase separation to occur. Typically, this involves letting thedispersion stand at ambient condition, preferably without agitation. Thephase separation forms a concentrated dispersion of typical solidscontent of about 35 wt % to about 70 wt %, preferably about 40 wt % toabout 70 wt % and more preferably about 50 wt % to about 70 wt %, as alower phase. The upper phase will have substantially lower solids,preferably less than about 1 wt %.

Another preferred method for concentration is electrodecantation.Concentration of solids by electrodecantation is achieved byelectrophoretic migration of PTFE particles employing direct currentpotential which is applied from electrodes at either end of a containercontaining the dispersion. Negatively charged PTFE particles surroundedby anionic dispersing agent move in the applied field toward the anode.Semi-permeable membrane barriers are suspended vertically in thecontainer between the electrodes and form concentrating cells andprevent the particles from contacting the anode. Concentrated dispersionmoves downward along one face of the membrane and depleted dispersionmoves upward on the other face. The concentrated dispersion being densersettles to the bottom of the container and can be drawn of at intervals.The supernatant liquid remains overhead.

Coating Applications

The invention further relates to a process for applying fluoropolymer toa substrate comprising providing a concentrated aqueous fluoropolymerdispersion comprising fluoropolymer particles and having a solidscontent of about 35 to about 70 wt %, the dispersion comprising about0.03 wt % to about 10 wt % anionic polyelectrolyte dispersing agentbased on the weight of fluoropolymer solids and containing less thanabout 300 ppm fluorosurfactant based on the weight of the dispersion,the dispersion being substantially free of nonionic dispersion; applyingthe dispersion to the substrate; and heating the substrate coated withdispersion to coalesce the fluoropolymer particles on the substrate.

The process of the invention is especially suitable for metalsubstrates, preferably for coating compositions which contain chromicacid as a component of a primer coating to increase adherence to themetal. One such application is the coating of catheter guide wire. Otherapplications include the coating of any of a variety of metal processequipment components such as rollers, tanks, trays, etc. Knowntechniques and equipment for coating application can be adapted for usein accordance with the process of the present invention.

For some applications, anionic surfactant such as ammonium or alkalimetal lauryl sulfate, e.g., sodium lauryl sulfate is added to thefluoropolymer dispersion of this invention. Addition of anionicsurfactant can lower the viscosity and enhance the stability of thedispersion. For coating uses, anionic surfactant can improve wettingcharacteristics. The anionic surfactant can be added can be added to thedispersion at any time after fluorosurfactant reduction. However, it isgenerally preferable to add the anionic surfactant after concentrationto prevent loss of the anionic surfactant with the water phase removedduring concentration. Preferably, the anionic surfactant is present inthe stabilized dispersion of this invention in an amount of about 0.05to about 5 wt % based on the weight of the fluoropolymer solids.

Test Methods

Solids content of raw (as polymerized) fluoropolymer dispersion aredetermined gravimetrically by evaporating a weighed aliquot ofdispersion to dryness, and weighing the dried solids. Solids content isstated in weight % based on combined weights of PTFE and water.Alternately solids content can be determined by using a hydrometer todetermine the specific gravity of the dispersion and then by referenceto a table relating specific gravity to solids content. (The table isconstructed from an algebraic expression derived from the density ofwater and density of as polymerized PTFE.)

Number average dispersion particle size on raw dispersion is measured byphoton correlation spectroscopy.

Standard specific gravity (SSG) of PTFE resin is measured by the methodof ASTM D-4895. If a surfactant is present, it can be removed by theextraction procedure in ASTM-D-4441 prior to determining SSG by ASTMD-4895.

Surfactant and solids content of stabilized dispersion are determinedgravimetrically by evaporating a small weighed aliquot of dispersion todryness following in general ASTM D-4441 but using a time andtemperature such that water but not the surfactant is evaporated. Thissample is then heated at 380° C. to remove the surfactant and reweighed.Surfactant content is stated in wt % based on fluoropolymer solids.

Gel time is measured by the time it takes a dispersion to completely gelin a blender. 200 ml of dispersion is placed in a Waring commercialexplosion resistant blender (Model 707SB, one quart size, run at highspeed, air requirements—10 scfm @ 10 psi, available from Waring of NewHartford, Conn.). This blender has a capacity of 1 liter and has an airpurge for the motor. The dispersion is stirred at the highest speeduntil the dispersion gels. The gel point is quite sharp and easy todetermine. The Gel Time is recorded is seconds. If the dispersion doesnot gel in ½ hour (1800 seconds), the test is terminated to avoid damageto the blender. The blender is then completely disassembled and cleanedafter each determination.

Fluorosurfactant Content is measured by a GC technique in which thefluorosurfactant is esterified with acidic methanol. Perfluoroheptanoicacid is used as an internal standard. Upon addition of electrolyte andhexane the ester is extracted into the upper hexane layer. The hexanelayer is analyzed by injection onto a glass GC column of 20 ft.×2 mmI.D. packed with 10% OV-210 on 70/80 mesh Chromosorb W.AW.DMCS. held at120 C. The detector is ECD and the carrier gas of 95% argon/5% methanehas a flow rate of 20 to 30 ml/min.

EXAMPLES Fluoropolymers

TFE is polymerized to produce a raw PTFE homopolymer dispersioncontaining PTFE particles having an SSG of about 2.20 and a numberaverage particle size of approximately 220 nm. The raw as-polymerizeddispersion contains approximately 45% fluoropolymer solids and has anAPFO content of about 1800 ppm. In order to determine Gel Times, rawas-polymerized dispersion is used as described in the examples below.

Dispersing Agents

TAMOL® 681 supplied by Rohm and Haas is the ammonium salt of a copolymercontaining about 39% methacrylic acid and about 61% butyl methacrylatewith a molecular weight of approximately 10,000. The dispersing agent isreceived as a very viscous liquid in water having a viscosity of 9700cps and contains 35% active ingredients. The copolymer is described asbeing hydrophobic.

TAMOL® 2001 supplied by Rohm and Haas is the sodium salt of a same asTAMOL® 681. The dispersing agent is received as milky white liquid inpropylene glycol having a viscosity of 20 cps with a pH of 3.4 andcontains 42% active ingredients. For effective use, ammonium hydroxideis added to form an ionized salt. Adding ammonium hydroxide to Tamol2001 results in a very high viscosity, approximately 38,000 cps. If theTamol is diluted to 20% or lower, the viscosity is less than 40 cps whenammonium hydroxide is added. Alternatively the Tamol can be mixed intothe dispersion and then ammonium hydroxide can be added.

TAMOL® 165A—Hydrophobic acrylic copolymer. TAMOL® 165A has a viscosityof 660 cps. This grades contains 21% active ingredients.

TAMOL® 731A—Hydrophobic acrylic copolymer. The TAMOL® 731A has aviscosity of 56 cps and contains 25% active ingredients.

TAMOL® 1124—Hydrophillic acrylic copolymer containing 50% activeingredients.

TAMOL® 963—A poly-acid dispersant containing 35% active ingredients.

Ion Exchange Resin

A244-OH by US Filter is commercially-available strong base anionexchange resin with quaternary ammonium groups with a dimethyl ethanolamine moiety in hydroxide form.

Example 1

Several samples of 200 grams of as-polymerized (raw) PTFE dispersion at41 wt % solids are placed in 8-ounce glass jars. The jars areapproximately ¾ full.

-   -   Sample 1 contains only as-polymerized PTFE dispersion.    -   Sample 2 contains as-polymerized PTFE dispersion stabilized with        1.22 wt % TAMOL® 681 (active ingredient basis) based on the dry        weight of PTFE.    -   Sample 3 contains as-polymerized PTFE dispersion with 6 wt % of        wet US Filter A-244-OH ion exchange resin    -   Sample 4 contains as-polymerized PTFE dispersion stabilized with        1.22 wt % TAMOL® 681 based on the dry weight of PTFE and 6 wt %        of wet US Filter A-244-OH ion exchange resin.

The jars are placed on a Brunnell Wrist Action Shaker and shaken on aspeed setting of 1 at room temperature, a suitable setting to get goodmixing of the ion exchange resin and PTFE dispersion in Samples 3 and 4.

-   -   Sample 1 containing only PTFE dispersion is ⅔ coagulated after        shaking for about 1.5 hours.    -   Sample 2 containing 1.22 wt % TAMOL® 681 based on the dry weight        of PTFE is unchanged visually after 2 hours of shaking        indicating that the TAMOL® 681 provided a degree of shear        stability.    -   Sample 3 containing ion exchange resin shows immediate clumping        as soon as ion exchange resin is added. After shaking one hour,        the pH rose from 3.5 to 4.1 and the conductivity fell from 1685        microsiemens per cm to 183 indicating that APFO removal is        taking place. After two hours the PTFE is completely coagulated        and the ion exchange resin is encapsulated by the PTFE gel.    -   Sample 4 containing 3.5% TAMOL® 681 and 6 wt % A-244-OH ion        exchange resin is visually unchanged after two hours of shaking,        i.e. no coagulation is present and the ion exchange resin still        floated on the surface of the dispersion. The pH rose and the        conductivity fell indicating that some APFO removal had        occurred.

Example 2 Shear Stability as Measured by Gel Time

To establish an effective level of TAMOL® 681, samples of as-polymerizedPTFE dispersion and TAMOL® 681 are sheared in a Waring blender at highspeed until the dispersion gelled. The resulting Gel Times are shownbelow in Table 1. The shear stability is found to be highly dependent onconcentration of TAMOL®. The wt % Tamol 681 is expressed on the basis ofactive ingredients relative to PTFE solids.

TABLE 1 wt % TAMOL ® 681 Gel Time, seconds 0.35 2 0.70 4 1.05 3 1.49 431.75 727

Example 3 Fluorosurfactant Removal

A dispersion containing 1.75 wt % TAMOL® 681 (active ingredients basis)in as-polymerized PTFE with 41 wt % solids is prepared. Additionalammonium hydroxide is added to insure the pH remains above 9.5. Varyinglevel of A-244-OH ion exchange resin are added and the samples areshaken on the Brunnell Wrist Action Shaker at a speed setting of 1 for 3hours. The samples are then analyzed for APFO levels based on the totaldispersion weight. The results are shown below.

TABLE 2 wt % A-244-OH Resin APFO, ppm 1.2 879 3.5 434 5.8 232 8.1 14910.5 99.8 12.8 64

The results indicate that the TAMOL® 681 provides sufficient stabilityto allow ion exchange to reduce the level of APFO in fluoropolymerdispersions. It is also a good indication that the ion exchange resin isnot removing the TAMOL® as well as the APFO.

The rate of ion exchange can be further improved by using additional ionexchange resin or by increasing the temperature at which the ionexchange is conducted.

Example 4

Alternate grades of TAMOL® are tested for shear stability inas-polymerized PTFE dispersion with APFO present. As will be seen below,hydrophobic acrylic copolymers exhibit good shear stability wherehydrophilic acrylic copolymers do not. Tests are conducted to determinethe minimum amount of the different grades to give adequate shearstability as shown in the Table below. As shown below, hydrophobiccopolymer dispersing agents exhibit desirable shear stability whereasthe hydrophilic copolymer dispersing agents and poly-acid grade do not.

TABLE 3 Gel Times with Different Grades of TAMOL ® TAMOL ® wt % TAMOL ®Type Active Ingred.* Gel Time, sec Description T-2001 1.79 385Hydrophobic T-2001 3.19 1042 Hydrophobic T-165A 2.43 >1800 HydrophobicT-165A 1.77 >1800 Hydrophobic T-165A 1.46 430 Hydrophobic T-731A2.43 >1800 Hydrophobic T-731A 1.76 509 Hydrophobic T-1124 3.19 2Hydrophilic T-1124 6.38 2 Hydrophilic T-963 3.19 0 poly acid, PTFEgelled when added *Based on PTFE

Example 5

This example illustrates reduction of fluorosurfactant-containingfluoropolymer dispersion containing acrylic copolymer dispersing agentsin an anion exchange column and subsequent concentration using acrylicpolymer of high acid content.

100 parts of as-polymerized PTFE (43.5 wt % solids) is mixed with 1.74parts TAMOL® 2001 (active ingredients basis). The dispersion is gentlystirred and ammonium hydroxide is added to bring the pH up to 9.8. Thedispersion is passed through a column containing US Filter A-244 OH ionexchange resin. The column is 14 inches in diameter and has a length todiameter ratio of 8:1. The temperature of the dispersion is maintainedat 52-54° C. The flow rate through the column is approximately 10 poundsper minute. This results in a reduction of the APFO level from 1600 ppmto 8.1 ppm.

After ion exchange, the solids level is measured at 41.2 wt %. Asolution of polyacrylic acid (PAA) is prepared to concentrate thedispersion. 42.7 grams of Aquatreat AR-7H (available from Alco Chemicalsas a 15% solution) are added to 157.3 grams water. Ammonium hydroxide isadded to raise the pH to 9.5 to convert the PAA to the ammonium salt(raising the viscosity of the stock solution from 13 cps to 238 cps)causing the PAA the be added to the dispersion in salt form. If added inthe acidic form, the PAA will lower the pH of the dispersion and theTAMOL® will become insoluble resulting in localized coagulation of thePTFE dispersion.

37.2 grams of the PAA stock solution is added to 2058 grams of theTAMOL® stabilized dispersion that has been subjected to the ion exchangeprocess. The mixture is heated to 75° C. with stirring and the stirreris then turned off to allow concentration to occur. After one hour, theupper layer is removed. The concentrated lower layer contains 66.0% PTFEsolids. The viscosity of the dispersion is 318 cps.

The concentrated dispersion is diluted to 60.0% solids withdemineralized water. This reduces the viscosity to 84 cps. 2.4% sodiumlauryl sulfate is added based on PTFE solids to improve wettingcharacteristics, reducing the viscosity further to 33 cps.

Example 6

This example illustrates reduction of fluorosurfactant-containingfluoropolymer dispersion containing acrylic copolymer dispersing agentsin an anion exchange column and subsequent concentration usingelectrodecantation.

Similar to Example 5, 100 parts of as-polymerized PTFE (43.5 wt %solids) is mixed with 1.74 parts TAMOL® 2001 (active ingredients basis).The dispersion is gently stirred and ammonium hydroxide is added tobring the pH up to 9.8. The dispersion is passed through a columncontaining US Filter A-244 OH ion exchange resin. The column is 14inches in diameter and has a length to diameter ratio of 8:1. Thetemperature of the dispersion is maintained at 52-54° C. The flow ratethrough the column is approximately 10 pounds per minute. This resultsin a reduction of the APFO level from 1600 ppm to 8.1 ppm.

After ion exchange, the solids level is measured at 41.2 wt %.Concentration of solids is performed by electrodecantation. Thedispersion is fed to a container and a direct current potential isapplied from electrodes at either end of the container (240 V d.c.).Negatively charged PTFE particles surrounded by anionic dispersing agentmove in the applied field toward the anode. A plurality of semipermeable membrane barriers suspended vertically in the containerbetween the electrodes form concentrating cells and prevent theparticles from contacting the anode. Concentrated dispersion movesdownward along one face of a membrane and depleted dispersion movesupward on the other face. Periodic current reversal prevents compactionand coagulation of the resin particles. The concentrated dispersionbeing denser settles to the sloping bottom of the container which is notseparated into concentrating cells. The concentrated dispersion iswithdrawn form the bottom of the decanter in 30 minute intervals. Thesupernatant liquid remains overhead. The applied current is alternatedevery 90 seconds to reverse the flow of the PTFE particles thuspreventing coagulation of PTFE powder on the membranes and electrodes.The concentrated lower layer contains 54.0% PTFE solids. The viscosityof the dispersion is approximately 30 cps.

Example 7

This example illustrates the coating process in accordance with theinvention for the application of fluoropolymer to catheter guide wire.Catheter guide wire, typically stainless steel, is coated withfluoropolymer to facilitate access into the arterial system.

The dispersion as prepared in Example 6 is used. Such dispersion isformulated into a coating composition as explained in U.S. Pat. No.2,562,118 to Osdal. The coating composition is formed from 100 parts ofPTFE dispersion containing about 1 wt % sodium lauryl sulfate blendedwith 35 parts of an acid accelerator system containing chromic acid andphosphoric acid. The coating composition is sprayed wet onto a stainlesssteel wire (outer diameter of 0.15 to 0.40 inches, 3.8 to 10 mm) whichhas been thoroughly cleaned and dried to remove all milling oils, dirt,etc., to a dry film thickness of 10-15 micrometers. The coating is curedby baking at a temperature of 750° F. (399° C.) for 3-5 minutes

Example 8

This example illustrates the coating process in accordance with theinvention for the application of fluoropolymer as an industrial finishsuch as would be applied, for example, to the inside of a chemicalprocessing tank.

The dispersion as prepared in Example 6 is further used to coat asubstrate of carbon steel. A substrate of carbon steel is roughened bygrit blasting with aluminum oxide to achieve a surface roughness Ra ofabout 75 to about 125 micrometers using a coarse grit (10-20 mesh) and90-100 psi (0.62-0.69 MPa) air pressure. The dispersion of Example 6 isformulated into a coating composition as explained in U.S. Pat. No.2,562,118 to Osdal and applied as a primer to the prepared substrate.The coating composition is formed from 100 parts of PTFE dispersioncontaining about 1 wt % sodium lauryl sulfate, blended with 35 parts ofan acid accelerator system containing chromic acid and phosphoric acid.The primer is applied wet by conventional methods to a film thickness ofabout 12 to 25 micrometers.

A first layer of PFA powder (Type 350, product code 532-5450manufactured by the DuPont Company) is applied electrostatically to thewet primer using a powder spray gun supplied by ITW GEMA Company. Gunsettings are 15 kV, 3.0 conveying air, 10 dosing air and 6 bar pressure.The first coating layer is applied onto the wet primer and athermocouple is attached to the coated substrate. The coated substrateis heated, and while heating, the temperature of the substrate ismeasured with the thermocouple and the coated substrate is baked for 10minutes at 725-750° F. (385-399° C.) The substrate is removed the ovenand the second and subsequent layers are applied. Each layer is appliedat about 80 to about 120 microns DFT per coat and the recoated substrateis then baked for 10 minutes at 700-725° F. (371-385° C.) The bakingtime can be extended if desired to insure complete melting andcoalescence of the coating. The substrate is coated repeatedly with PFApowder composition until a DFT in the range of 625 micrometers isreached.

1. An aqueous fluoropolymer dispersion comprising fluoropolymerparticles having an average particle size of about 10 to about 400 nm,said dispersion having a solids content of about 35 to about 70 wt % andcomprising about 0.03 wt % to about 10 wt % anionic polyelectrolytedispersing agent based on the weight of fluoropolymer solids, saiddispersion being substantially free of nonionic surfactant and having aGel Time of at least about 100 seconds.
 2. The dispersion of claim 1further comprising acrylic polymer of acid content of 20% or more byweight.
 3. The dispersion of claim 1 wherein said fluoropolymerparticles have an average particle size of about 100 to about 350 nm. 4.The dispersion of claim 1 wherein the solids content of the dispersionis about 50 to about 70 wt %.
 5. The dispersion of claim 1 wherein saidanionic polyelectrolyte dispersing agent is present in an amount ofabout 0.1 wt % to about 10 wt % based on the weight of fluoropolymersolids.
 6. The dispersion of claim 2 wherein said acrylic polymer ofacid content of 20% or more is present in an amount of about 0.01 toabout 0.5 wt % based the weight of the aqueous portion of thedispersion.
 7. The dispersion of claim 2 wherein said acrylic polymer ofacid content of 20% or more is present in an amount of about 0.02 toabout 0.4 wt % based the weight of the aqueous portion of thedispersion.
 8. The dispersion of claim 1 wherein said dispersioncontains less than about 300 ppm fluorosurfactant based on the weight ofsaid dispersion
 9. The dispersion of claim 1 wherein said dispersioncontains less than about 100 ppm fluorosurfactant based on the weight ofsaid dispersion.
 10. The dispersion of claim 1 wherein said dispersioncontains less than about 50 ppm fluorosurfactant based on the weight ofsaid dispersion.
 11. The dispersion of claim 1 wherein said dispersionhas a Gel Time of at least 300 seconds.
 12. The dispersion of claim 1wherein said dispersion has a Gel Time of at least 500 seconds.
 13. Thedispersion of claim 1 further comprising anionic surfactant.
 14. Thedispersion of claim 13 wherein said anionic surfactant is present in anamount of about 0.05 to about 10 wt % based on the weight offluoropolymer solids.
 15. The dispersion of claim 1 having a pH of atleast about 9.0.
 16. The dispersion of claim 1 having a pH of at leastabout 9.5.
 17. The dispersion of claim 1 having a surface tension ofgreater than about 35 mS/cm.
 18. The dispersion of claim 1 wherein theanionic polyelectrolyte dispersing agent is an anionic polymer, having alinear or branched structure, with anionic groups distributed along thepolymeric chain.
 19. The dispersion of claim 1 wherein the anionicpolyelectrolyte dispersing agent has an equivalent weight greater thanabout
 150. 20. The dispersion of claim 1 wherein the equivalent weightof the anionic polyelectrolyte dispersing agent is less than about50,000.
 21. The dispersion of claim 1 wherein the number averagemolecular weight of the anionic polyelectrolyte dispersing agent is atleast about
 500. 22. The dispersion of claim 1 wherein the anionicpolyelectrolyte dispersing agent contains at least 2 anionic functionalgroups per molecule.
 23. The dispersion of claim 1 wherein the anionicpolyelectrolyte dispersing agent contains anionic functional groupsselected from carboxylates, sulphates, sulphonates, phosphates, andphosphonates.
 24. The dispersion of claims 1 wherein the anionicpolyelectrolyte dispersing agent does not contain fluorine atoms. 25.The dispersion of claims 1 wherein the anionic polyelectrolytedispersing agents are selected from anionic homopolymers or copolymersof monomers selected from acrylic or vinyl monomers.
 26. The dispersionof claim 25 wherein the acrylic monomers are selected from(meth)acrylamide, (meth)acrylic acid in the form of the correspondingsalts, (meth)acrylonitrile, linear or branched C₁-C₄ hydroxyesters ofthe (meth)acrylic acid, C₁-C₁₂ alkyl(meth)acrylates, wherein the alkylcan be linear or branched, compounds of the following general formula:

wherein: R₁ is H or CH₃; R₂ and R₃, equal or different, are H or C₁-C₈alkyl, optionally branched; M is an alkaline or alkaline-earth metal orammonium and A is NH, O or NCH₃.
 27. The dispersion of claim 25 whereinsaid anionic polyelectrolyte dispersing agent is an acrylic copolymer.28. The dispersion of claim 27 wherein said anionic polyelectrolyedispersing agent is present in an amount of about 0.5 wt % to about 5.5wt % based on the weight of fluoropolymer solids.
 29. The dispersion ofclaim 27 wherein said acrylic copolymer dispersing agent has a molecularweight of about 2,000 to about 100,000.
 30. The dispersion of claim 27wherein said acrylic copolymer dispersing agent is hydrophobic.
 31. Thedispersion of claim 27 wherein said acrylic copolymer dispersing agentcomprises methacrylic acid/butyl methacrylate copolymer.
 32. Thedispersion of claim 31 wherein said methacrylic acid/butyl methacrylatecopolymer comprises about 30 to about 50 mol % methacrylic acid unitsand about 50 to about 70 mol % butyl methacrylate units.
 33. Thedispersion of claim 25 wherein the vinyl monomers are selected fromvinylaromatic monomers, C₁-C₁₂ alkyl vinylethers, and vinyl esters ofC₁-C₁₈ aliphatic monocarboxylic acids.
 34. The dispersion of claim 33wherein said vinylaromatic monomers are selected from styrene and itsderivatives obtained by substituting one or more hydrogen atoms of thearomatic ring with a hydroxyl or a methyl and/or of vinyl with a methyl.35. The dispersion of claim 1 wherein the anionic polyelectrolytedispersing agent comprise polyamic acids.
 36. The dispersion of claim 35wherein the anionic polyelectrolyte dispersing agent comprise polyamicacids selected from aromatic polyamic acids or polyamidoamic acids. 37.The dispersion of claim 35 wherein the polyamic acids contain thefollowing units:

wherein R^(II) is a divalent arylene radical.
 38. The dispersion ofclaims 1 wherein the anionic polyelectrolyte dispersing agent comprisecarboxyalkyl celluloses, wherein alkyl comprises from 1 to 5 carbonatoms.
 39. A process for reducing the fluorosurfactant content of andconcentrating an aqueous fluorosurfactant-containing fluoropolymerdispersion comprising: stabilizing fluoropolymer dispersion comprisingfluoropolymer particles having an average size of about 10 to about 400nm and a solids content of about 15 to about 55 wt % with anionicpolyelectrolyte dispersing agent; contacting said stabilizedfluorosurfactant-containing aqueous fluoropolymer dispersion with ananion exchange resin to reduce fluorosurfactant content to apredetermined level; separating said anion exchange resin from saiddispersion after the fluorosurfactant content has been reduced toproduce a reduced-fluorosurfactant dispersion; and concentrating saiddispersion to at least about 35% by weight using a method which producesdispersion substantially free of nonionic surfactants.
 40. The processof claim 39 wherein said concentrating is performed by adding acrylicpolymer of acid content of 20% or more by weight to saidreduced-fluorosurfactant dispersion in an amount of about 0.01 to about1 wt % based on aqueous content of said dispersion and subjecting saiddispersion to conditions causing said dispersion to separate into alower phase high in fluoropolymer solids and an upper phase low influoropolymer solids and recovering said lower phase as concentratedreduced fluorosurfactant dispersion.
 41. The process of claim 40 whereinthe pH of said stabilized fluoropolymer dispersion is at least about 6when said adding of acrylic polymer of acid content of 20% or more isperformed.
 42. The process of claim 40 wherein the pH of said stabilizedfluoropolymer dispersion is at least about 9 when said adding of acrylicpolymer of acid content of 20% or more is performed.
 43. The process ofclaim 40 wherein the pH of said stabilized fluoropolymer dispersion isat least about 9.5 when said adding of acrylic polymer of acid contentof 20% or more is performed.
 44. The process of claim 39 wherein saidconcentrating is performed by electrodecantation.
 45. The process ofclaim 39 wherein said concentrated reduced fluorosurfactant dispersionrecovered has a solids content of about 35 to about 70 wt %.
 46. Theprocess of claim 39 wherein said concentrated reduced fluorosurfactantdispersion recovered comprises less than about 300 ppm fluorosurfactant.47. A process for applying fluoropolymer to a substrate comprising:providing a concentrated aqueous fluoropolymer dispersion comprisingfluoropolymer particles and having a solids content of about 35 to about70 wt %, said dispersion comprising about 0.03 wt % to about 10 wt %anionic polyelectrolyte dispersing agent based on the weight offluoropolymer solids and containing less than about 300 ppmfluorosurfactant based on the weight of said dispersion, said dispersionbeing substantially free of nonionic dispersion; applying saiddispersion to said substrate; and heating said substrate coated withdispersion to coalesce said fluoropolymer particles on said substrate.48. The process of claim 47 wherein said substrate is metal.
 49. Theprocess of claim 47 wherein said aqueous dispersion further compriseschromic acid.
 50. The process of claim 47 wherein said substrate iscatheter guide wire.
 51. A coating composition comprising the dispersionof claim 1 further comprising chromic acid.